In quantum communication systems, information is transmitted between a sender and a receiver by encoded single quanta, such as single photons. Each photon carries one bit of information encoded upon a property of the photon, e.g. polarisation, phase or energy/time of the photon. The photon may even carry more than one bit of information, for example, by using properties such as angular momentum.
An important application of quantum communications is for quantum key distribution which is a technique for forming a shared cryptographic key between two parties; a sender, often referred to as “Alice”, and a receiver often referred to as “Bob”. The attraction of this technique is that it provides a test of whether any part of the key can be known to an unauthorised eavesdropper (Eve). In many forms of quantum key distribution, Alice and Bob use two or more non-orthogonal bases in which to encode the bit values. The laws of quantum mechanics dictate that measurement of the photons by Eve without prior knowledge of the encoding basis causes an unavoidable change to the state of the photons. These changes to the states of the photon will causes errors in the bit values sent between Alice and Bob. By comparing a part of their communication, Alice and Bob can thus determine if Eve has gained information.
In order for such a system to operate correctly, it is important that each bit is encoded upon just one photon. This may be achieved using a single photon source, for which at most only one photon is generated within each photon pulse output. Alternatively, a weak pulsed laser may be used, in which case the laser pulses are attenuated to the level where the average number of photons per photon pulse is less than 1.
Both of these types of photon source are inefficient, i.e. some output pulses will not contain a photon and will thus not carry any information. In the case of a single photon source, the emitting material may be inefficient and sometimes not generate a photon at the desired time and/or the collection of the emitted photons into the following apparatus may be inefficient. Both of these effects will reduce the rate of useful photon pulses generated by the device. For the case of weak laser pulses, it is necessary to set the attenuator so that the average number of photons per output pulse is much less than 1, so as to reduce the rate of multiple photon pulses. This inefficiency of the photon source has an adverse effect upon the bit rate of the communication system.
The photons sent by Alice to Bob are encoded by an appropriate apparatus and sent to Bob over a communications medium such as a fibre optic cable or through free space. Photons will be lost in Alice's and Bob's apparatus due to inefficiency of its components, or while in transit from Alice to Bob over the communication medium. This loss further reduces the bit rate.
In order to maximise the bit rate, it is advantageous to increase the frequency of attempting to send photons from Alice to Bob. However, this is constrained by the maximum rate at which the single photon detector can be operated. In particular many types of single photon detector can register a false positive reading shortly after a correct positive reading has been registered. Avalanche photodiodes operated in Geiger mode for single photon detection are particularly prone to false positives being. registered after a true reading has been registered.
In order to address this problem, the detector is gated so that the detector is only on for the time when a photon is expected and then switched off. The detector is switched off for long enough to ensure that it does not register a false positive reading when it is switched backed on. This requirement further decreases the possible bit rate of the communication system.